Method for producing glass or glass ceramic and in particular glass or glass ceramic article

ABSTRACT

In order to obtain glass or glass ceramic materials having increased strength, a method is provided for producing glass or glass ceramic articles, which comprises: producing an initial glass body, mounting the initial glass body on a gas cushion between a levitation support and the initial glass body, and at least partially ceramizing the initial glass body on the levitation support. The levitation support comprises at least one continuous surface region having at least one gas feed region where levitation gas for the gas cushion is fed out from the levitation support, and at least one gas discharge region where gas from the gas cushion is at least partially discharged into the levitation support.

CROSS-REFERENCE TO RELATED APPLICATIONS

Patent application U.S. Ser. No. 11/720,915, filed Jun. 5, 2007, andPatent application DE 10 2004 059 728.6, filed Dec. 11, 2004, areincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates in general to the production of glass or glassceramics and in particular to the production of glass or glass ceramicsby mounting on a gas bed during ceramization, as well as to glass orglass ceramic articles produced according to the method.

BACKGROUND OF THE INVENTION

Glass ceramic plates are employed inter alia extensively as cookingsurfaces for modern hobs, as oven windowpanes or fireproof glazing. Withglass ceramics, the strength and surface condition plays an essentialrole for their fields of use.

Glass ceramics used for hobs currently often have a knopped undersidestructure, in order to increase the strength of the glass ceramic platesfor hob applications.

In particular, the knops provide protection of the underside of theceramic plate against strength-reducing damage, which is incurredespecially during the process of ceramizing the glass ceramic.

The strength is finally achieved by the knops absorbing damage to thelower sides.

A disadvantage with such knopped glass ceramic plates, however, is interalia the scattering of light which is sent through the glass ceramicplate. It is therefore impossible, or possible only with difficulty, fordisplays or structures below the glass ceramic plate to be made visiblewithout distortion.

In the past, glass ceramic hob surfaces which have smooth surfaces onboth sides have also been produced for hobs. With a plate thickness of 4mm, an average strength value of 36 cm fall height was achieved in atest format of 10×10 cm.

In modern ceramizing methods, strengths of up to 50 cm fall height areachieved for 4 mm thick glass ceramic plates smooth on both sides. Suchglass ceramic hob surfaces smooth on both sides with a sufficientstrength have previously been obtained by re-polishing the glass ceramicplates. This process cannot be carried out economically for industrialproduction.

For the application of glass ceramic plates for stove and ovenwindowpanes, it is likewise necessary on the one hand to achieve acertain strength. On the other hand, the surfaces of the glass ceramicshould be free of damage, since damage to the surface impairs theoptical transmission and the esthetic impression. Damage furthermoreconstitutes potential sites where the glass can easily break by impact.

The glass ceramic plates currently used primarily comprise damage due tothe ceramization process, which has a visually perturbing effect andsignificantly reduces the strength.

The strength properties are determined by the quality of the surface. Ina known ceramization process, a green glass plate to be ceramized isplaced on a ceramic support plate, in which case separating means may beused between the green glass plate and the support plate. In this case,the glass ceramics which have been ceramized on a support generallycomprise damage on the side that faces the support plate.

Another method uses suspended ceramization. In this method, the greenglass plate is mounted in suspension at one end. Although the glassceramic is mounted contact-free in suspended ceramization, so that nodamage can occur on a support, it is nevertheless difficult to achieve aplanarity of the ceramic glass plates that meets requirements.Furthermore, the entire area of the glass ceramics ceramized insuspension cannot be utilized, since the plates become damaged in theregion of the suspension points.

Another way of producing high-strength glass ceramics is provided bychemically prestressing the article after ceramization. In this regard,see patent specification DE 1803540. A disadvantage with this method,however, is that a further process step is necessary in which the glassceramic article additionally needs to be heated. The chemicalprestressing method can furthermore be applied only, as indicated in thepatent specification, to very particular compositions.

It is known from GB 1383202 to mount a plate to be ceramized on a gascushion between a support and the plate. To this end, the supportcomprises perforated tiles, through which the gas for the gas cushion isdelivered. The gas delivered through the perforated tiles flows betweenthe plate and the support to the edge of the plate, where it finallyescapes. The effect of this arrangement, however, is that onlycomparatively small glass plates can be ceramized. For large glassplates, the pressure which is created under the glass plate is toogreat, so that curvature of the plate takes place.

Furthermore, GB 1383202 proposes to use combustion gas as a levitationgas for the gas cushion. This, however, is disadvantageous for theproperties of the glass ceramic, since the combustion gases on the onehand contain particles which are preferentially deposited on the glassceramic and therefore lead to a strength reduction. Likewise, readilyscaling metals and combustion products in the oven space likewise leadto the glass plate being contaminated on its surface.

DE 10045479 describes a method for the contactless mounting andtransporting of flat glass, which likewise involves mounting on a gascushion. To this end the support has a segmented structure, in which thegas is supplied through openings in the segments and can escape againthrough the gaps between the segments.

U.S. Pat. No. 5,078,775 describes a gas cushion support with adiaphragm, the upper side of which has slotted gas feed and gas outletopenings. The gas outlet openings are in communication with gas outletchambers in the diaphragm. The gas feed openings are connected viamanifold shafts to the lower side of the diaphragm. On the lower side ofthe diaphragm, compressed gas is supplied which flows through themanifold shafts and the gas feed openings to the upper side, where itgenerates a gas cushion on which a glass plate can then be mounted. Withsuch an arrangement in which the feed gas is supplied perpendicularly tothe upper side, however, the gas comes only comparatively briefly incontact with the walls of the manifold containers, so that full heatexchange does not take place and the gas can therefore flow into the gascushion at a temperature which may differ from the temperature of thediaphragm and in particular the temperature of the supported glass.

SUMMARY OF THE INVENTION

It is an object of the invention to provide glass or glass ceramicmaterials, which in particular have a smooth fire-polished and/orknopped surface, the materials having a significantly increased strengthcompared with known glass ceramics together with less opticallyperturbing damage of the surface.

This object is directly achieved in a very surprisingly simple way by amethod for producing a glass or glass ceramic article as well as a glassor glass ceramic article.

Accordingly, the method according to the invention comprises

-   -   producing an initial glass body,    -   mounting the initial glass body on a gas cushion between a        levitation support and the initial glass body, and    -   at least partially ceramizing the initial glass body on the        levitation support. The levitation support has at least one        continuous surface region with at least one gas feed region        where levitation gas for the gas cushion is fed out from the        levitation support, and at least one gas discharge region where        gas from the gas cushion is at least partially discharged into        the levitation support.

In contrast to the device disclosed in U.S. Pat. No. 5,078,775, in whichthe gas is applied via manifold shafts through the diaphragm, accordingto the invention the gas which is supplied to the gas cushion ispreferably also delivered through one or more chambers arranged in thelevitation support, before it emerges therefrom. In this way, inconjunction with the gas discharge chamber or chambers, not only aparticularly homogeneous pressure distribution but also good temperatureequilibration is achieved. Owing to the residence time of the gas in thegas feed, its temperature is matched better to the temperature of thediaphragm and of the mounted glass plate or glass ceramic plate. Thisarrangement and procedure according to the invention for mounting theglass or the glass ceramic helps to achieve a very uniform temperaturedistribution along the mounted plate. This also leads in a surprisinglysimple way to an increased strength of glass ceramic plates which aremounted according to the invention during the ceramization.

For effective heat transfer between the levitation gas flowing out ofthe diaphragm and that flowing into it, it is advantageous for the atleast one gas feed chamber and at least one gas discharge chamber tocomprise closed channels, which extend in the direction along thebearing surface inside the diaphragm. The term closed channel isaccordingly intended to mean a channel which is bounded by a wall in themanner of a tube. The channels may in particular be configured so thatthey are closed or at least partially closed from the bottom surface ofthe levitation support, which lies opposite the bearing surface. Aclosed channel in the context of the invention does not mean a fullysealed cavity, however, since at least one inlet opening for the gasfeed chamber and one outlet opening for the gas discharge chamber areprovided for the feed and discharge from the chambers or respectively tothe chambers of this embodiment.

The gas feed chamber may advantageously have a gas inlet opening and thegas discharge chamber a gas outlet opening, which are arranged so thatthe gas flow direction inside the gas feed chamber and the gas dischargechamber extends transversely to the normal of the bearing surface, andin particular along the bearing surface. If the levitation gas in thegas feed chamber and the gas discharge chamber in the diaphragm flowstransversely to the normal of the bearing surface, particularly in thedirection along the bearing surface, then a long flow path of the gas inthe chambers and concomitantly also effective heat transfer to thechamber walls can be achieved even in a thin diaphragm as the levitationsupport.

According to yet another preferred embodiment of the invention, thelevitation gas is fed into the gas feed chamber through at least onegas-permeable connection on the lower side of the diaphragm, or thelevitation support. An antechamber, preferably with a ceramic wall,arranged below the gas feed and gas discharge chambers, may then beprovided which is connected via at least one gas-permeable connection tothe gas feed chamber in order to feed gas into the gas feed chamber. Itwould also be conceivable for the antechamber itself to be a componentof the diaphragm or levitation support, or to use an integral diaphragmhaving at least one antechamber, gas feed chamber and gas dischargechamber. In each case, the gas feed chamber is at least partially closedat the bottom, the levitation gas being introduced into the gas feedchamber by a downwardly directed gas-permeable connection in thediaphragm into the gas feed chamber. Although the gas is also suppliedfrom below, for example in the gas cushion support known from U.S. Pat.No. 5,078,775, this diaphragm does not however have chambers or channelsfor the gas feed which are closed or at least partially closed at thebottom. Rather, the channels are entirely open at the bottom. Incontrast to this, the effect achieved by the gas feed chamber with agas-permeable connection, particularly in the form of channels with asmall cross section as the gas feed chamber, is that the gas resides fora longer time in the gas feed chamber before it emerges from the bearingsurface via the further gas-permeable connection. In this embodiment ofthe invention as well, improved heat exchange with the diaphragm andtherefore particularly good temperature homogeneity in the gas cushionare therefore achieved. In particular, with a device according to theinvention, the temperature of the levitation cushion can be kept with atemperature gradient of less than 10° C., preferably less than 5° C. inthe direction along the bearing surface by means of the diaphragm withchambers for gas feed and gas discharge. Likewise conceivable, albeitsomewhat more elaborate, is a reversed configuration in which it is notthe gas feed chambers but the gas discharge chambers that arecorrespondingly attached to an antechamber by means of a downwardlydirected gas-permeable connection, so that the gas from the levitationcushion travels via the gas discharge chambers into the antechamber andis discharged there.

The pressure drop in the chambers is preferably at most 0.5 mbar. If thelevitation gas experiences a pressure drop of at most 0.5 mbar whenflowing through the gas feed chamber and/or the gas discharge chamber,then a particularly homogeneous pressure distribution can also beachieved in the gas cushion.

The initial glass body may be produced by a conventional melting andshaping process, before the initial glass body is then ceramizedaccording to the invention for example in a levitation oven.

The least partial ceramization may in particular also comprisenucleation. It is important to mount the glass or the glass ceramic on alevitation cushion on the one hand whenever the glass or the glassceramic becomes very soft and/or whenever the glass or glass ceramicplate expands or contracts strongly. In the conventional ceramizationprocess, the latter leads to a relative movement between the glass plateand the support plate, the effect of which is that scratches are formedon the more sensitive glass plate. For ceramization, the glass isinitially brought to a nucleation temperature. This nucleationtemperature lies at a temperature for which the glass plate reachesviscosities in the region of from ç=10¹⁰ dPas. It is therefore preciselyduring this phase that mounting on a maximally homogeneous pressureprofile is favorable for the properties, in particular planarity andstrength, of the article finally produced.

It is furthermore advantageous for the glass to have no contact with thesupport at said viscosities. If contact of the initial glass body with asupport takes place, then adhesion of the glass to the support mayoccur.

During the ceramization process, this nucleation phase is followed by acrystal growth phase.

By the method according to the invention, a particularly homogeneoustemperature profile can also be achieved both along the surface of theinitial glass body and between the upper and lower sides of the body,since there is no longer any contact with the support. The supportotherwise represents a heat reservoir which can adapt only slowly andinhomogeneously to the temperature changes occurring during theceramization.

Particularly during the crystal formation of phase, the temperaturehomogeneity is important for the future quality and strength of theceramized glass, so that levitational mounting is particularlyadvantageous here.

The crystal growth may furthermore take place very rapidly with aparticular temperature adjustment. Because of this, so much heat isreleased inside the glass plate that the initial glass body becomesubstantially softer and viscosities of ç=10⁸ dPas or less are reached.Even at such low viscosities, damage-free processing is possible withlevitational mounting according to the invention on a gas consumed.

The least partial ceramization also need not comprise full ceramization.For example, the material of the initial glass may be onlysemi-ceramized in order to obtain desired physical properties of thearticle finally produced.

In the course of the ceramization process, the glass plate may change inits geometrical dimensions. This effect often occurs, for example,because of the phase transition during ceramization. This may involveboth shrinkage and expansion of the glass plate. These changes oftenoccur both in the nucleation range and in various phases of the crystalgrowth. In the conventional ceramization method, a strong relativemovement of the glass plate relative to the support plate occurs inthese phases, which may lead to scratches in the product. According toan advantageous refinements of the method according to the invention,therefore, the glass plate or the initial glass body is mountedlevitating on the glass cushion while it shrinks or expands.

By mounting on a levitation support with a continuous surface, whichcomprises regions in which gas is supplied and discharged, a previouslyunachieved homogeneous pressure distribution is achieved under theinitial glass body. This leads to particularly little deformation oreven no longer any deformation of the initial glass, even though itgenerally becomes very soft during the ceramization, in which caseviscosities of 10⁸ dPas or even lower may be reached. Owing to thehomogeneous pressure distribution, furthermore, essentially no tensileor compressive stresses occur any longer during the ceramization.Concomitantly with a homogeneous pressure distribution, a particularlyhomogeneous pressure distribution in the initial glass can furthermorebe achieved by the method according to the invention during theceramization. The substantially larger pressure gradients otherwiseoccurring under the material in known methods lead to gas flowsextending laterally in the direction of the gradients. If thetemperature of the gas does not correspond accurately to the temperatureof the initial glass, then heat will be dissipated or supplied via thelocally differing gas flows. A homogeneous temperature distributionunder and over the initial glass, however, is important for theceramization in the initial glass and the planarity. In particular,temperature differences even in the range of a few degrees can lead tocurvature of the glass.

It is furthermore advantageous for the levitation gas to be at leastpartially recirculated. In this way, a circuit of the levitation gas isachieved. This is advantageous particularly when the glass or the glassceramic is mounted in the hot state on the gas cushion, for example forceramization. The recirculated gas is thus already heated when it entersthe gas feed chambers, so that the levitation support is cooled onlylittle by the supplied levitation gas. This on the one hand savesenergy, and on the other hand the homogeneity of the temperaturedistribution is perturbed little or not at all. For temperaturedifferences which are as small as possible, it is furthermoreadvantageous for the levitation gas to be taken from the environment ofthe initial glass body, for example the oven space of a ceramizing oven,in which the levitation support is arranged.

Compared with an article produced by suspended separation, a glass orglass ceramic article produced according to the invention is furthermoredistinguished by a planer surface. With suspended ceramization, in thesoftened state of the initial glass body, flow of the material can takeplace in the gravitational direction i.e. along the surface of theinitial glass body, which leads to a significantly inhomogeneousthickness of the finally ceramized article. Restoring forces in theevent of curvatures, with a ceramized initial glass body lying accordingto the invention on a plane support, are also much higher than with aninitial glass body mounted in suspension. In the ceramization accordingto the invention, the surface of the initial glass body matches thesurface of the support, so that undesired curvatures are compensatedfor. This effect does not occur with a freely suspended body, however,so that curvatures may remain.

According to one embodiment of the invention, surface-wide ceramizationof the initial glass body is carried out so that a glass wall glassceramic article ceramized surface-wide is obtained. This is possibleowing to the levitational support, since no or only minimal holding orguiding is necessary in order to hold or guide the initial glass body.Conversely, for example in suspended ceramization, no ceramization canbe carried out in the holding region since the material there becomestoo soft or significant damage occurs.

Devices which are suitable for carrying out the method according to theinvention, i.e. for producing glass or glass ceramic articles accordingto the invention, are also described in the Applicant's Germanapplication filed on the same day as the present invention with thetitle “Method and Device for the Contactless Transport or Support ofGlass or Glass Ceramic”, the disclosure of which is also fullyincorporated in the subject-matter of the present invention.

The production of the initial or preliminary glass body mayadvantageously also comprise the separation of sections from an initialglass web. The separated sections may then be ceramized separately. Thisavoids subsequent coating of the ceramized material, which may causedamage reducing strength in the glass ceramic.

Conversely, the crystallization process in the initial glass is notperturbed by the method according to the invention. This correspondinglyleads to a glass ceramic, or a glass or glass ceramic article producibleby the method according to the invention, having novel and surprisingproperties. A glass or glass ceramic article, which is producible by themethod according to the invention, is intended to mean an article havinga material which may comprise glass and/or in particular glass ceramicand/or semi-ceramized glass.

The glass ceramic articles producible according to the invention, havean increased strength without chemical prestressing, so thatprestressing is obviated according to one embodiment of the inventionchemical, i.e. the a glass ceramic article according to the invention isnot chemically prestressed therein.

In order to characterize the strength of the glass or glass ceramicarticles produced according to the invention, standardized droppingtests may be carried out as a strength measurement. In this case theplate to be tested is cut into samples with a defined format (100 mm×100mm) or produced in this defined format, and tested by means of a balldropping test. The ball dropping test is carried out by letting a steelball with a defined mass (m=200 g) and a defined diameter (Ø 36 mm) fora freely onto the center of the sample from an initial height. If thesample survives this fall without breaking, then the fall is repeatedwith an increased fall height. This iterative method, with a fall heightrespectively increased in a defined way, is carried out until breakingof the sample occurs. The fall height at which breaking occurs is takenas a measure of the strength of this sample. The strength of the entireplate to be tested, or a batch of produced articles, is given by theaverage value of the individual strengths of the samples cut from it.

Owing to the production method, and article producible according to theinvention thus in particular has a significantly increased breakingstrength compared with known glass ceramic materials, which ismanifested by a correspondingly increased average fall height in thetest described above. This increased strength is already exhibited byglass ceramic articles which are not additionally prestressed, inparticular without chemical prestressing. The glass or glass ceramicarticles according to the invention are thus distinguished by an averagebreaking fall height which is at least 15 cm per millimeter thickness ofthe glass or glass ceramic article with a format of 10×10 cm. An averagebreaking fall height of 18 cm per millimeter thickness of the glass orglass ceramic article is also readily achieved or exceeded. These valueswere verified particularly in a thickness range of between 3 and 5millimeters of the material of the article, but also apply for othermaterial thicknesses above and below this range since there is anessentially linear relation between breaking strength and averagebreaking fall height.

According to initial discoveries, the increased strength of the glassceramic articles producible according to the invention is due inter aliato a vitreous film forming on the surface of the article.

In general 20 cm per millimeter thickness, or even at least an 25 cmaverage breaking fall height per millimeter thickness of the ceramicarticle are also achieved. Furthermore, and even average breaking fallheights of 30 or more centimeters per millimeter of plate thickness areachievable by optimizing the production parameters.

These advantageous strength properties of the glass ceramic articlesproducible according to the invention make it possible to reduce thethickness for an equal strength compared with conventionally producedarticles, which inter alia lowers the material costs and therefore theprice of such articles.

The linear relation between plate thickness and breaking fall heightapplies so long as the glass plate has sufficient opportunity to absorbthe impact of the ball by bending. In the case of particularly thickglass plates, however, the plate may break earlier. A departure from thelinear relation may take place for thick glass plates in the rangebeyond about 10 mm thickness, in which case the gradient of the averagevalue of the fall height then generally decreases with an increasingplate thickness. The linear relationship of the breaking fall height andthe plate thickness, which applies in wide ranges, is known inter aliafrom J. L. Glathart, F. W. Preston: “The behaviour of glass underimpact”; in: Glass Technology, 1968.

A glass or glass ceramic article producible according to the inventionmay in particular comprise a material that is breakproof with a fallheight of more than 45 centimeters in the form of a 3 millimeter thickplate plane on both sides with the format 10×10 cm. In general, breakingfall heights of at least 60 cm are even achieved.

In particular, it is also possible to produce a glass or glass ceramicarticle according to the invention so that its material is breakproof onaverage with a fall height of at least 80 centimeters in the form of a 3millimeter thick plate plane on both sides.

According to another embodiment of the glass or glass ceramic articleaccording to the invention, it comprises a material that is breakproofon average with a fall height of at least 140 centimeters in the form ofa 5 millimeter thick plate plane on both sides. This surpasses thestrength of known glass ceramic articles without chemical stressing,measured from the fall height, by more than a factor of 2.

The values specified above relate to material with a particular shapeand thickness. This does not generally mean that the glass or glassceramic article per se must have a format of 10×10 centimeters, ratherthat a test body cut with the respectively indicated thicknesses fromthe glass or glass ceramic article has said average breaking fallheights. A glass or glass ceramic article according to the invention mayaccordingly have many other shapes and also other thicknesses.

The specifications regarding strength serve in particular tocharacterize the material of the article, but not its shape andthickness. If an article according to the invention has a shape which isnot plate-like or a different thickness, then, in order to determine itsmechanical properties by a dropping test, one or more plates with adefined thickness may be produced and ceramized according to theinvention from the same initial glass in order to carry out the droppingtest. For example, strength values may be obtained in a simple way forvarious plate thicknesses by interpolating the values specified above.According to one embodiment of the invention, the article according tothe invention therefore comprises a material that is breakproof onaverage with a fall height of at least x centimeters at least 60centimeters in the form of a thick plate plane on both sides with theformat 10×10 cm and with a thickness in the range of from 3 to 5millimeters in a steel ball dropping test with a steel ball of mass 200g, where x is given by the interpolation relation: x=(140 cm−45 cm)/2mm*(plate thickness in mm−3 mm)+45 cm.

According to one embodiment of the method according to the invention, apreliminary glass body is produced in the form of a glass plate plane onboth sides, for example with a thickness of 3 or 5 millimeters, andsubsequently at least partially ceramized levitationally, so that acorresponding glass or glass ceramic articles move on both sides isobtained. Such articles are suitable for example as a glass ceramichotplate, a stop windowpane, as flameproof or fireproof panes. Thelevitationally carried out ceramization process according to theinvention provides a fire-polished surface of the article. Compared witha surface subsequently polished mechanically, the fire-polished surfacehas substantially less or even known surface damage, which also leads toan increased strength of the article according to the invention comparedwith such subsequently polished plates. Glass or glass ceramic articlesproducible according to the invention may therefore be used as securityglazing. Such security glazing may in particular be armored glass, oreven bulletproof glass.

Knopped plates, as are often manufactured to date in order to achievesufficiently high breaking safety, may also be produced andlevitationally ceramized die the method according to the invention. Tothis end, a corresponding preliminary or initial glass body is producedparticularly in the form of a glass plate knopped on one side. For thisembodiment of the method according to the invention, a somewhat highergas flow with a constant floating height is generally needed when theknops lie on the side facing the diaphragm. The knopped structure may,for example, be impressed in a melting and shaping process via a knoppedroller during molding in one side of a preliminary glass web, inparticular the lower side of the glass web. The knop structure may havea regular pattern of pimples, which are round or oval, or otherprojections.

Such a glass or glass ceramic article producible according to theinvention, which corresponds in its external shape to a conventionallyproduced knopped plate, also has a breaking safety increased by at least20% compared with such a conventionally produced knopped plate.

A glass or glass ceramic article according to the inventionadvantageously comprises a material of at least one of the systemsSiO₂—Al₂O₂—Li₂O, SiO₂—Al₂O₂—MgO, SiO₂—Al₂O₂—BaO.

The material may furthermore comprise at least one of the oxides TiO₂,ZrO₂, P₂O₅ in a conventional concentration, in order to influence themechanical and optical properties and the viscosity of the initialglass.

The production of an initial glass body may advantageously comprisefining of the initial glass. An essentially bubble-free initial glass isobtained by the fining, which makes a considerable contribution to thestrength of the articles according to the invention. To this end theinitial glass may be supplemented with fining agents such as As₂O₂,Sb₂O₂, CeO₂ or SnO₂, which are therefore also found in the material ofthe article finally produced.

Particularly in order to influence the optical properties, for instantin order to impart the desired coloration to the article according tothe invention, at least one coloring oxide may be added to the materialof the initial glass.

According to one embodiment of the invention, the material of thefinished article or of the initial glass has the following components:

Li₂O 2.5-5.5%,   Na₂O 0-3.0%, K₂O 0-3.0%, ΣNa₂O + K₂O 0-4.0%, MgO0-3.0%, CaO 0-2.5%, SrO 0-2%,   BaO 0-3.5%, ZnO 0-3.5%, Al₂O₃ 18-27%,  SiO₂ 52-75%,   TiO₂ 1.0-5.5%,   ZrO₂ 0-3.0%, SnO₂ <1.0%, ΣTiO₂ + ZrO₂ +SnO₂ 2.0-6.0%,   P₂O₅ 0-8.0%.

The quantity specifications are given as weight proportions in percentby weight. The summation sign “Σ” denotes the sum of the substancequantities of the components listed after the summation sign.

According to a second embodiment of the glass or glass ceramic article,its material has one of the following compositions:

Glass 1 Glass 2 Component: Proportion: Component: Proportion SiO₂ 63-67,SiO₂ 65-69, preferably 65.5 preferably 67.5 Al₂O₃ 22 to 24, l_(h)e_(a)t19 19 to 21, preferably 23.2 preferably 20 Li₂O 2.5-4, Na₂O0-0.5, preferably 3.3 preferably 0.1 Na₂O 0-0.5, MgO 0.5-1.5, preferably0.4 preferably 1.1 MgO 0.2-0.8, BaO 0.5-1.5, preferably 0.5 preferably0.9 BaO 0-0.5, ZnO 1.5-2, preferably 0.05 preferably 1.6 ZnO 0-0.5, ZrO₂1.5-2, preferably 0.05 preferably 1.8 ZrO₂ 2-2.5, TiO₂ 2-3, preferably2.2 preferably 2.7 TiO₂ 1.5-2.5, As₂O₃ 0.5-1, preferably 2.09 preferably0.81 As₂O₃ 1.0-2.5, K₂O 0-0.5, preferably 1.13 preferably 0.2 P₂O₅0.5-1.5, preferably 1.3 K₂O 0-0.5, preferably 0.3 V₂O₃ 0-0.1, preferably0.03

The proportions in the table above are likewise given in percent byweight.

A dark coloration, as is often desired for hobs, may advantageously beachieved by a composition of the material of the article or the initialglass whose composition comprises from 0.02 to 0.6% by weight of V₂O₅.For a transparent article, correspondingly, a composition which isessentially free of V₂O₅ may advantageously be selected.

The composition of the initial glass wall the finally ceramized materialmay furthermore advantageously comprise at least one compound from agroup which comprises Cr, Mn, Fe, Co, Cu, Ni, Se, Cl compounds. Suchcompounds are suitable in particular to support the coloration andadjust particular color loci.

According to one embodiment of the method according to the invention,the ceramization of the initial glass body comprises particularly cleanelectrical heating of the initial glass body, preferably in a levitationoven.

The levitation gas may also be purified, for example by means of apurifying filter, and thus kept as clean as possible in order to preventthe precipitation of extraneous substances and avoid a reduction in thestrength due to this.

The levitation support may advantageously comprise at least onediaphragm with a continuous surface region and at least one gas feedchamber and at least one gas discharge chamber. These chambers arepreferably arranged respectively below the gas feed and gas dischargeregions. In this way, gas is supplied to the gas feed region via a gasfeed chamber and forms a gas cushion between the continuous surfaceregion and the initial glass body. On the other hand, excess gas canpass through the gas discharge region into a gas discharge chamberarranged underneath. The gas feed and gas discharge can be adjusted bysetting up a suitable pressure gradient between the gas dischargechamber and the gas discharge region, are respectively between the gasfeed chamber and the gas feed region. In order to achieve this, apressure gradient may be set up in a straightforward way between the gasfeed and gas discharge chambers. A suitably shaped diaphragm withchambers may, in particular, be produced by extrusion.

According to a preferred embodiment of the invention, the gas is fed toa perforated surface of the levitation support and discharged via feedand discharge channels. The surface of the levitation support mayadvantageously also comprise a porous material, through which gas forthe gas cushion is fed or discharged. The levitation gas is transportedby the air supply system through the diaphragm into the gas cushion,which is formed between the diaphragm and the glass plate. The pressureunder the initial glass is stabilized by the air discharge system, whichcomprises in particular the gas discharge region and the gas dischargechamber. Owing to the gas film stabilized in this way, the initial glassbody then flows over the diaphragm and is thus mounted contact-free.

The extruded diaphragm is perforated, so that a maximally homogeneouspressure profile is formed. This homogeneous pressure profile isadvantageous in order to achieve a high planarity of the glass or glassceramic articles produced according to the invention.

The desired temperature profile for the ceramization with as small aspossible a temperature difference between the upper and lower sides ofthe glass, can advantageously be produced by temperature homogenizationof the levitation gas taking place in the gas feed chambers of theextruded diaphragm. The initial glass body may be partially or fullymounted on the glass film during the ceramization process.

According to one embodiment of the method according to the invention, aparticular chronological temperature profile is executed during theceramization process. For this temperature profile, the initial glassbody is first heated to a temperature T1. This temperature lies forexample in a range of from 650 to 800° C. The body may be kept at thistemperature for up to 4 hours, depending on the oven unit and the shapeof the initial glass body. The body is subsequently heated further to atemperature T2 in a range of from 850 to 950° C. The body may then bekept at this temperature T2 for up to about 50 minutes. The body issubsequently cooled again to room temperature.

As well as flat glass ceramic plates, for example, glass or glassceramic articles with a curved plate-like plate may also be producedaccording to the invention. To this end, the levitation support may havea curved surface. The initial glass body in the heated state can then becurved by gravitational sinking over the levitation support. It isadvantageous in this case for levitation gas to be supplied during thecurvature process, so that the curvature can take place contact-free.According to a refinement of the invention, the article has curvaturealong one direction, i.e. uniaxial curvature. Such an article may forexample comprise a uniform curvature, so that it has the shape of acylinder lateral surface segment. Nevertheless, the radius of curvaturemay also change along the surface. The production of such a shapedarticle may be carried out as described above, by gravitational sinkingover a diaphragm which in this case is uniaxially curved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail below with reference tothe appended figures. Parts which are the same and similar are providedwith the same references in the figures.

FIG. 1 shows a cross-sectional view through a levitation support of adevice for carrying out the method according to the invention,

FIG. 2 shows a plan view of an embodiment of a levitation support,

FIG. 3 shows a refinement of the embodiment shown in FIGS. 1 and 2,

FIGS. 4A and 4B show, with the aid of views of an embodiment of a devicefor carrying out the method according to the invention, the method stepsfor producing a curved glass or glass ceramic article,

FIGS. 4C and 4D show with the aid of schematic views of anotherembodiment of a device for carrying out the method according to theinvention, the method steps for producing a curved article, and

FIG. 5 shows the dependency of the average breaking fall height on thethickness of glass ceramic articles.

DETAILED DESCRIPTION

FIG. 1 represents a cross-sectional view of a levitation support in theform of a diaphragm denoted overall by 1. The diaphragm is preferablyproduced by extruding a suitable material, such as a refractory ceramic.The diaphragm 1 has a number of chambers 51, 52, 53, 71, 72, 73, whichare arranged in the diaphragm below the continuous bearing surface 3arranged on one side of the diaphragm. The chambers 51, 52, 53 are asgas feed chambers and the chambers 71, 72, 73 are used as gas dischargechambers for the levitation gas of the gas cushion, which is formedbetween an initial glass body to be ceramized and the continuous bearingsurface 3 of the diaphragm by supplying the levitation gas via thechambers 51, 52, 53. For illustration, FIG. 1 shows an initial glassbody 11, lying in levitation above the surface region 3 on a gas cushionor gas film 13, in the form of a plate which is plane on both sides.

The surface region 3 is perforated and has gas feed channels 91, 92, 93and gas discharge channels 101, 102, 103 as perforations. The levitationgas is supplied and discharged via the gas feed channels 91, 92, 93 andgas discharge channels 101, 102, 103 which are in communication with thesurface region 1 and the chambers 51, 52, 53 and 71, 72, 73. The gasflow direction is represented by arrows in FIG. 1. To this end apressure difference is generated between the chambers 51, 52, 53 and 71,72, 73, a high pressure being set up in the gas feed chambers 51, 52, 53than in the gas discharge chambers 71, 72, 73. For the ceramization, theinitial glass body may then for example be heated electrically in orderto avoid precipitation. By a filter (not shown), the levitation gas maybe purified before entering the gas feed chambers so that, for example,it is essentially free of suspended particles that may accumulate on thesurface of the initial glass body in its softened state.

The levitation gas may advantageously be recycled by a correspondinginstrument. To this end the gas is taken from the environment of theinitial glass body 11, for instance an oven space in which the diaphragmis arranged, and fed back to the diaphragm 1 so as to achieve goodtemperature equilibration between the gas cushion and the environment ofthe levitationally mounted initial glass body 11.

The initial glass body 11 is mounted on the gas cushion 13 over thediaphragm 1 in particular whenever it shrinks or expands and/or has lowviscosities, at which the initial glass body could otherwise adhere tothe support. Shrinkage and growth processes take place particularlyduring the crystal growth. Levitational mounting during the crystalgrowth phase is also generally favorable for the quality of the articlesproduced according to the invention, since by mounting on the gascushion it is possible to achieve a particularly homogeneous temperaturedistribution along the surface minimal temperature differences betweenthe upper and lower sides of the glass body.

During the ceramization, the temperature of the levitation cushion iskept with a temperature gradient of less than 10° C., preferably lessthan 5° C., in the direction along the bearing surface of the levitationsupport. Particularly uniform ceramization is achieved by this uniformtemperature distribution.

FIG. 2 shows a plan view of the continuous surface of one embodiment ofa levitation support 1 in the form of a diaphragm. The surface region 3of the levitation support 1 has gas feed regions 151, 152, 153 at whichlevitation gas for the gas cushion is supplied, and gas dischargeregions 171, 172, 173 at which gas from the gas cushion is at leastpartially discharged. The regions 151, 152, 153, 171, 172, 173 aremarked by the dashed lines in FIG. 2. The gas is supplied via gas feedchannels 95 arranged in the gas feed regions 151, 152, 153, and the gasis discharged via gas discharge channels 105 arranged in the gasdischarge regions. For the sake of clarity, only a few of the channels95 and 105 are labeled in FIG. 2. As represented in FIG. 1, the channels95 and 105 are connected to chambers arranged below the surface region3. In particular, the chambers 51-53, 71—are formed as closed channelswhich extend in the direction along the bearing surface on the inside ofthe diaphragm 3 below the gas feed and the gas discharge regions. Thechannels are also closed off at least partially from the bottom surfaceof the diaphragm 1, which lies opposite the bearing surface for theinitial gas 11, so that there is a thermal barrier to the lower side ofthe diaphragm 1.

FIG. 3 represents a refinement of the invention, in which the gas isintroduced into the gas feed chambers via an antechamber 6 arrangedbelow the diaphragm 1. In this refinement of the invention as well, thegas feed chambers 51, 52, 53 are bounded at least partially at thebottom by a wall of the diaphragm 1. The gas feed chambers 51, 52, 53respectively have downwardly directed gas-permeable connections in theform of channels 96 on the lower side or bottom surface 4, for supplyinggas from the antechamber 6. The antechamber 6 is formed by anantechamber housing 5 attached to the bottom surface 4. The antechamberhousing 5 is preferably made from ceramic material like the diaphragm 1,in order to avoid contamination of the levitation gas.

By means of the diaphragm according to the invention, as has beendescribed by way of example with the aid of FIGS. 2 to 3, the glass orthe glass ceramic 11 is kept by the levitation cushion at a height of atleast 750 micrometers, preferably up to at most 2 millimeters, above thebearing surface.

The method steps for producing a curved glass or glass ceramic articleare represented in FIGS. 4A and 4B as well as 4C and 4D by way ofexample with the aid of a schematic view of two embodiments of a devicefor carrying out the method according to the invention. First, aplate-like initial glass body 11 is formed in the conventional way bymolding. This is subsequently put into a levitation of an 19 with alevitation support arranged therein in the form of a diaphragm 1, sothat the initial glass body 11 lies above the surface region 3 asrepresented in FIG. 4A. The diaphragm 1 is constructed similarly asrepresented in FIGS. 1 to 3. In contrast to the embodiments shown inFIGS. 1 to 3, the embodiment of the diaphragm 1 shown in FIGS. 4A to 4Dhas a curved surface region 3. This is represented a convexly curvedsurface region by way of example in FIGS. 4A and 4B. Concave curvatureor a combination of convexly and concavely curved regions, for exampleas in a corrugated surface, are however likewise possible. FIGS. 4C and4D show an embodiment with a concavely curved surface region.

In order to keep the initial glass body floating above the diaphragm 1,so as to avoid contact with the support 1, a gas cushion between thesupport and the initial glass body 11 is generated by gas supply via thegas feed regions of the surface region 3. Advantageously, the initialglass body may also be slightly laterally held or guided, in order toprevent the body from drifting away. Such holding or guiding requiresonly minimal holding forces during the levitational support. The pointsof contact with the holding or guiding instrument can therefore be keptvery small, so that surface-wide ceramization of the initial glass bodyis achieved.

Subsequently, by means of an electrical heating device 21 arranged inthe levitation oven, the initial glass body is heated until it softens.Owing to the force of gravity acting on the initial glass body, itlikewise becomes curved, the regions of the initial glass body lyingfurther away from the levitation support 1 sinking until an essentiallyhomogeneous pressure distribution is produced by the gas cushion. Thissituation is shown by FIGS. 4B and 4D, respectively. The ceramization onthe support 1 may be incorporated with the curving process or carriedout subsequently. Particularly during the nucleation process, the glassgenerally becomes very soft and can easily be curved by gravitationalsinking during this phase.

According to one embodiment of the invention, the diaphragm comprises asurface curved uniaxially, i.e. along one direction, so that glassceramic articles correspondingly curved uniaxially are obtained.

The properties of glasses or glass ceramic articles produced accordingto the invention will be explained further by way of example below withthe aid of application examples.

For a first plate-like article produced according to the invention witha thickness of 5 millimeters, the strength values achieved were onaverage with a breaking strength at a fall height of more than 140centimeters. The composition of the glass ceramic material of thisarticle corresponded to glass type 2. This value for a 5 mm thick glassplate exceeds by more than two times the values of 60 cm fall height,which are otherwise conventionally measured for non-prestressed glassceramic plates of this thickness.

For another 3 millimeter thick plate-like glass ceramic article with acomposition of the glass ceramic corresponding to glass type 1, anaverage breaking fall height of more than 80 cm was found. An averagebreaking fall height of at least 55 centimeter or 18 cm per millimeterthickness of the glass ceramic is readily achieved or even greatlyexceeded, as shown by the example above, for plate-like glass ceramicarticles produced according to the invention.

Here as well, the fall height achievable for the average breakingstrength is about two times as great as the fall height of about 40 cmotherwise achievable with known glass ceramic articles.

The measured values are listed again more accurately in the followingtable.

Fall height of Fall height of the average the average breaking strengthbreaking strength [cm] for glass [cm] for ceramic producedconventionally Glass Thickness according to the produced glass system[mm] invention ceramic Glass 1 3 mm 87 40 Glass 2 5 mm 142 63

With the aid of this table, it is clear that the plates producedaccording to the invention achieve a significantly higher strength owingto the novel method of ceramization on a levitation support with acontinuous surface and gas feed and gas discharge regions and the highpressure homogeneity thereby achieved in the gas cushion and the uniformtemperature distribution in the initial glass body. The measurementvalues indicated were achieved without additional chemical prestressing.

Reference will be made below to FIG. 5, which shows the dependency ofthe average breaking fall height of the thick glass ceramic articleswith the aid of measured values. Conventionally produced glass ceramicplates and glass ceramic articles according to the invention arerespectively represented. With the aid of this diagram, it is againclear that in terms of strength a glass ceramic article producedaccording to the invention far surpasses a glass ceramic articleceramized conventionally while resting on a support and in contact withthe support.

With the aid of the measurement values of the conventionally producedplates and FIG. 5, the linear correlation of the plate thickness withthe average breaking fall height can also be seen. Thus, according tothis linear relation, the conventionally produced glass ceramic platestested have an average breaking fall height of about 12.5 cm permillimeter of plate thickness, corresponding to the straight linedenoted by “A”. Conversely, the measurement values shown in the FIG. 5for the tested glass ceramic plates according to the invention showed anaverage breaking fall height of about 28.5 cm per millimeter of platethickness. These achieved strength values lie above the minimum valuesachievable by their production method according to the invention, namely15, 18, 20 or 25 centimeters average breaking fall height per millimeterof plate thickness. The plates for the dropping test were ceramized inan experimental arrangement. The temperature homogeneity and thehomogeneity of the pressure profile can be improved further in anindustrial application, so that average breaking fall heights of 30 cmper millimeter of plate thickness or even more are possible.

The considerably increased strength of plates produced according to theinvention was demonstrated in the exemplary embodiments with the platethicknesses 3 mm and 5 mm. It is clear to the person skilled in the artthat plates produced according to the invention with a different platethickness, for example between 3 mm and 5 mm, also have acorrespondingly increased strength. The increased to strength of platesproduced according to the invention with thicknesses of between 3 mm and5 mm can be interpolated with the aid of relevant literature. (See forexample: J. L. Glathart, F. W. Preston: “The behaviour of glass underimpact”; in: Glass Technology, 1968). Accordingly, for plate thicknessesof between 3 mm and 5 mm, increased strengths are obtained which can beinterpolated linearly from the strength values of the exemplaryembodiments.

It is clear to the person skilled in the art that the invention is notrestricted to the exemplary embodiments described above, but may bemodified in a variety of ways. In particular, the features of theindividual exemplary embodiments may also be combined with one another.

What is claimed is:
 1. A glass or glass ceramic article produced inaccordance with a process comprising the steps of: producing an initialglass body; mounting the initial glass body on a gas cushion between alevitation support and the initial glass body; and at least partiallyceramizing the initial glass body on the levitation support; wherein thelevitation support has at least one continuous surface region with atleast one gas feed region where levitation gas for the gas cushion isfed out from the levitation support, and at least one gas dischargeregion where gas from the gas cushion is at least partially dischargedinto the levitation support.
 2. The glass or glass ceramic article asclaimed in claim 1, wherein the glass or glass ceramic article has anaverage breaking fall height which is at least 15 cm per millimeterthickness of the glass or glass ceramic article in a range of between 3and 5 millimeters thickness with the format 10×10 cm and a steel balldropping test with a steel ball of mass 200 g.
 3. The glass or glassceramic article as claimed in claim 1, wherein the glass or glassceramic article has an average breaking fall height which is at least 20cm per millimeter thickness of the glass or glass ceramic article in arange of between 3 and 5 millimeters thickness with the format 10×10 cmand a steel ball dropping test with a steel ball of mass 200 g.
 4. Theglass or glass ceramic article as claimed in claim 1, wherein the glassor glass ceramic article has an average breaking fall height which is atleast 30 cm per millimeter thickness of the glass or glass ceramicarticle in a range of between 3 and 5 millimeters thickness with theformat 10×10 cm and a steel ball dropping test with a steel ball of mass200 g.
 5. The glass or glass ceramic article as claimed in claim 1,comprising a material that is breakproof on average with a fall heightof more than 45 centimeters in the form of a 3 millimeter thick plateplane on both sides with the format 10×10 cm and in a steel balldropping test with a steel ball of mass 200 g.
 6. The glass or glassceramic article as claimed in claim 1, comprising a material that isbreakproof on average with a fall height of at least 60 centimeters inthe form of a 3 millimeter thick plate plane on both sides with theformat 10×10 cm and in a steel ball dropping test with a steel ball ofmass 200 g.
 7. The glass or glass ceramic article as claimed in claim 1,comprising a material that is breakproof on average with a fall heightof at least 80 centimeters in the form of a 3 millimeter thick plateplane on both sides with the format 10×10 cm and in a steel balldropping test with a steel ball of mass 200 g.
 8. The glass or glassceramic article as claimed in claim 1, comprising a material that isbreakproof on average with a fall height of at least 140 centimeters inthe form of a 5 millimeter thick plate plane on both sides with theformat 10×10 cm and in a steel ball dropping test with a steel ball ofmass 200 g.
 9. The glass or glass ceramic article as claimed in claim 1,comprising a material that is breakproof on average with a fall heightof at least x centimeters at least 60 centimeters in the form of a thickplate plane on both sides with the format 10×10 cm and with a thicknessin the range of from 3 to 5 millimeters in a steel ball dropping testwith a steel ball of mass 200 g, where x is given by the interpolationrelation: x=(140 cm−55 cm)/2 mm*(plate thickness in mm−3 mm)+55 cm. 10.The glass or glass ceramic article as claimed in claim 1, comprising aplate which is smooth on both sides.
 11. The glass or glass ceramicarticle as claimed in claim 1, comprising a plate which is knopped onone side.
 12. The glass or glass ceramic article as claimed in claim 1,wherein the article has a fire-polished surface.
 13. The glass or glassceramic article as claimed in claim 1, comprising a curved plate. 14.The glass or glass ceramic article as claimed in claim 13, wherein theglass or glass ceramic article has uniaxial curvature.
 15. The glass orglass ceramic article as claimed in claim 1, comprising a material of atleast one of the systems:SiO₂—Al₂O₃—Li₂O,SiO₂—Al₂O₃—MgO,SiO₂—Al₂O₃—BaO.
 16. The glass or glass ceramic article as claimed inclaim 1, comprising a material that has at least one of the oxides TiO₂,ZrO₂, P₂O₅.
 17. The glass or glass ceramic article as claimed in claim1, comprising a material that has the following components in percent byweight: Li₂O 2.5-5.5%,   Na₂O 0-3.0%, K₂O 0-3.0%, ΣNa₂O + K₂O 0-4.0%,MgO 0-3.0%, CaO 0-2.5%, SrO 0-2%,   BaO 0-3.5%, ZnO 0-3.5%, Al₂O₃18-27%,   SiO₂ 52-75%,   TiO₂ 1.0-5.5%,   ZrO₂ 0-3.0%, SnO₂ <1.0%,ΣTiO₂ + ZrO₂ + SnO₂ 2.0-6.0%,   P₂O₅ 0-8.0%.


18. The glass or glass ceramic article as claimed in claim 1, comprisinga material that has the following components: Component: Proportion:SiO₂ 63-67, Al₂O₃ 22 to 24, Li₂O 2.5-4, Na₂O 0-0.5, MgO 0.2-0.8, BaO0-0.5, ZnO 0-0.5, ZrO₂ 2-2.5, TiO₂ 1.5-2.5, As₂O₃ 1.0-2.5, P₂O₅ 0.5-1.5,K₂O 0-0.5, V₂O₃ 0-0.1,

or the following components: Component: Proportion: SiO₂ 65-69, Al₂O₃ 19to 21, Na₂O 0-0.5, MgO 0.5-1.5, BaO 0.5-1.5, ZnO 1.5-2, ZrO₂ 1.5-2, TiO₂2-3, As₂O₃ 0.5-1, K₂O 0-0.5,

in percent by weight.
 19. The glass or glass ceramic article as claimedin claim 1, comprising a material that has at least one of the finingagents As₂O₃, Sb₂O₃, CeO₂, SnO₂.
 20. The glass or glass ceramic articleas claimed in claim 1, comprising a material that has at least onecoloring oxide.
 21. The glass or glass ceramic article as claimed inclaim 1, comprising a material whose composition comprises from 0.02 to0.6 percent by weight of V₂O₅.
 22. The glass or glass ceramic article asclaimed in claim 1, comprising a material whose composition has at leastone compound from a group which comprises Cr, Mn, Fe, Co, Cu, Ni, Se, Clcompounds.
 23. The glass or glass ceramic article as claimed in claim 1,wherein the glass ceramic article is ceramized surface-wide.
 24. Theglass or glass ceramic article as claimed in claim 1, wherein the glassceramic article is not chemically prestressed.
 25. A hotplate comprisinga glass or glass ceramic article as claimed in claim
 1. 26. A flameproofpane or fireproof glazing comprising a glass or glass ceramic article asclaimed in claim
 1. 27. A stove windowpane comprising a glass or glassceramic article as claimed in claim
 1. 28. Security glazing, inparticular armored glass, particularly preferably bulletproof armoredglass, comprising a glass or glass ceramic article as claimed in claim1.